In the semiconductor industry, bipolar junction transistors ("BJT") are known and used in either amplifying or switching devices. When employed in the former device, a BJT typically amplifies a small alternating current ("AC") signal according to the BJT's current gain characteristics.
BJTs essentially comprise three doped active regions--an emitter, a base and a collector region. These regions form a first diode between the base and emitter and a second diode between the base and collector. Each diode junction can be forward or reversed biased depending on the external voltage applied thereto. Thus, the BJT is termed "forward active" when the emitter base junction diode is forward biased, while the collector base junction diode is reversed biased.
It has been observed that each active region has an associated current--I.sub.e, I.sub.b, and I.sub.c. Of these currents, with respect to an PNP-type device, I.sub.b is a minority carrier current involving the flow of electrons, while both I.sub.e and I.sub.c are majority carrier currents which pertain to the flow of holes. Similarly, in an NPN-type device, I.sub.b is a majority carrier current and both I.sub.e and I.sub.c are minority carrier currents. Further, the total emitter current comprises the sum of the total number of electrons reaching the collector and the total number of electrons that flow out of the transistor through the base. This can be expressed in mathematical terms by the following equation: EQU I.sub.e =I.sub.b +I.sub.c.
Further, the current gain relationship between I.sub.c and I.sub.b can be quantified by the following formula: EQU .beta.=I.sub.c /I.sub.b.
Thus, given these relationships, the extent of minority carrier flow directly relates to the value of the current gain.
Traditionally, the values of .beta. have provided a current gain in the range of 100 to 300. Factors, such as the doped conductivity configuration of the BJT, the degree of doping, and reliability of the device itself, all impact on the current gain's value. Therefore, a need exists for a method and structure for enhancing the current gain of BJTs, without effecting the dopant concentration of the overall device or its reliability.